In the past, there has been known a process for producing cis-epoxysuccinates, which comprises reacting maleates with hydrogen peroxide in an aqueous solution in the presence of an epoxidation catalyst such as tungstate or molybdate (G. B. Payne, J. Org. Chem. 24, 54, 1959). That process, however, when industrially applied, has suffered from the disadvantages of failing to avoid a large admixture of catalyst in the produced cis-epoxysuccinates, leading to not only the loss of the product quality but also the insufficiency of the recycling of an expensive catalyst, and as a result, it has seemed difficult of industrial practice also from the economical point of view.
There has also been proposed an improved process therefor, which is intended for producing an acid salts of cis-epoxysuccinic acid having a smaller water solubility than that of a normal salts of cis-epoxysuccinic acid, so that the catalyst may be more easily separated therefrom (Japanese Patent Publication (KOKOKU) No. 51-20490 and Japanese Patent Publication (KOKOKU) No. 54-29486). However, according to these prior processes, since pH of the reaction solution is in the acid range, the produced acid salts of cis-epoxysuccinic acid are further hydrolyzed to be converted to DL-tartrate. The production of DL-tartrate not only may bring about the loss of the product quality as well as that of the yield per unit weight of the starting maleate and hydrogen peroxide, but also the produced DL-tartrate may act as an epoxidation inhibitor. Consequently, it becomes necessary that such DL-tartrate is eliminated by any means, thereby disadvantageously complicating the catalyst recycling step. There can be considered an improved process therefor, wherein pH of the reaction solution is maintained in the neutral range. In such case, however, the yield of the acid salts of cis-epoxysuccinic acid may be remarkably decreased.